(1) Field of the Invention
The present invention relates to fabrication of semiconductor devices. More specifically, the present invention relates to fabrication of Metal Oxide Semiconductor Field Effect Transistors (MOSFET).
(2) Background Information
In MOSFETs with shorter channels there are effects, such as the short-channel effect, that cause significant deviations from the expected behavior of MOSFETs with long-channel. Short-channel effects become a dominant part of MOSFETs' behavior when the channel's length decreases below 2 micrometers. For example, it is known that when the channel's length of a MOSFET is on the order of a submicron or less, the short-channel effect influences the threshold voltage towards 0. As a result, the short-channel effect causes an increase in the leakage current when the transistor is in the cutoff condition.
Currently, the well doping profile with respect to the source and the drain of a MOSFET is symmetric, i.e. the dopings of the source and drain are substantially equal. In ultra large scale integrated (USLI) devices scaling regime, there is an intrinsic trade-off between the saturated drive current and the off-state current (leakage current) that depends on diffusion. To reduce the off-state current, higher well/channel doping is needed. This high doping increases the channel's resistance of the MOSFET causing reduction of the saturated drive current. There is a problem with the reduction of the off-state current when such reduction is at the expense of the saturated drive current.